1. Field of the Invention
The present invention relates generally to a method for dynamically allocating a bandwidth in a passive optical network, and in particular, to a method for allocating a bandwidth over which an optical line termination (OLT) transmits data to optical network units (ONUs) on the basis of a time axis in a Gigabit Ethernet passive optical network (GE-PON).
2. Description of the Related Art
In order to build a network of communications, various network structures, such as xDSL (x-Digital Subscriber Line), HFC (Hybrid Fiber Coax), FTTB (Fiber To The Building), FTTC (Fiber To The Curb) and FTTH (Fiber To The Home), and numerous evolution schemes have been proposed. Of the various network structures, FTTx (x=B, C, H) can be classified into an active FTTx implemented by an active optical network (AON) structure and a passive FTTx implemented by a passive optical network (PON) structure. Due to its network configuration of a point-to-multipoint topology using a passive element, the PON is preferred as it purports to provide an economical network.
In the PON, one optical line termination (OLT) is connected to a plurality of optical network units (ONUs) via a 1×N passive optical distribution network (ODN). In particular, the PON forms a tree-structured decentralized topology between the OLT and the ONUs.
For the PON, an asynchronous transfer mode passive optical network (ATM-PON) has been first developed and standardized. Recently, ITU-T (International Telecommunication Union-Telecommunication section) has published the standardization contents of the ATM-PON in ITU-T G.982, ITU-T G.983.1 and ITU-T G.983.3. In addition, IEEE802.3ah TF of IEEE (Institute of Electrical and Electronics Engineers) is conducting a standardization work on a Gigabit Ethernet passive optical network (GE-PON) system.
Medium access control (MAC) technology for the point-to-point Gigabit Ethernet and ATM-PON has already been standardized, and the contents thereof are disclosed in IEEE 802.3z and ITU-T G983.1. In addition, MAC technology in an ATM-PON is well disclosed in U.S. Pat. No. 5,973,374, issued on 2 Nov. 1999, entitled “Protocol for Data Communication over a Point-to-Multipoint Passive Optical Network,” invented by Gigad Ghaib et al.
FIG. 1 is a block diagram illustrating an example of a common ATM-PON structure. As illustrated, an ATM-PON includes one OLT 10 and a plurality of ONUs 12a to 12c, wherein the OLT 10 is connected to the ONUs 12a to 12c via an ODN 16.
The OLT 10 plays a central role of providing information to the subscribers of an access network. The OLT 10 is connected to the ODN 16, which distributes a downstream data frame transmitted from the OLT 10 to the three ONUs 12a to 12c of the tree topology structure and transmits upstream data frames from the ONUs 12a to 12c to the OLT 10. The ONUs 12a to 12c receive the downstream data frame and provide the received downstream data frame to their end users 14a to 14c. Further, the ONUs 12a to 12c transmit upstream data frames output from the end users 14a to 14c to the OLT 10 via the ODN 16. The three end users 14a to 14c may be of different termination devices that can be used in a PON, including a network terminal (NT).
In the ATM-PON, data frames each consisting of 53-byte ATM cells are transmitted in upstream and downstream directions. In the tree-type PON structure of FIG. 1, during downstream transmission, the OLT 10 properly inserts downstream cells to be distributed to the ONUs 12a to 12c into a downstream frame. During upstream transmission, the OLT 10 accesses data transmitted from the ONUs 12a to 12c by time division multiplexing (TDM) scheme. Here, the ODN 16 connected between the OLT 10 and the ONUs 12a to 12c is a passive element.
The OLT 10 performs a virtual distance correction using a so-called ranging algorithm so as to prevent data from colliding in the ODN 16, which is a passive element. In addition, during downstream data transmission from the OLT 10 to the ONUs 12a to 12c, the OLT 10 exchanges encryption keys for security and OAM (Operations, Administration and Maintenance) messages with the ONUs 12a to 12c. To this end, upstream and downstream frames have a corresponding data field prepared in a dedicated ATM cell or a general ATM cell over which messages can be exchanged at stated periods.
Currently, with the development of Internet technology, subscribers require a wider bandwidth. Accordingly, research efforts have been focused on end-to-end transmission used in Gigabit Ethernet technology that is capable of securing a high bandwidth (of about 1 Gbps) at a relatively low price rather than in ATM technology, which requires high-priced equipments, segmentation of IP(Internet Protocol) packet, and only provides a limited bandwidth (of a maximum of 622 Mbps).
In the case of the Gigabit Ethernet, a MAC protocol of a point-to-point scheme and a collision scheme have already been standardized and a MAC controller chip is commercially available. However, in the case of a point-to-multipoint GE-PON structure, its overall scheduling procedure including MAC control is currently being standardized in IEEE 802.3ah EFM (Ethernet in the First Mile) TF.
The ATM-PON conducts a dynamic bandwidth allocation (DBA) for data transmission using the ONUs. In such an ATM-PON, the ONUs have four independent queues according to the service class defined by such parameters as VBR (Variable Bit Rate), CBR (Constant Bit Rate) and real-time feature. QoS (Quality of Service) is secured by storing data traffics applied to the ONUs in their queues and dynamically allocating a bandwidth considering the service class.
However, unlike the ATM, the GE-PON has an Ethernet-based protocol, so there is no service class. In the GE-PON based on Ethernet, a packet size is variable, so its bandwidth allocation method is distinguished from the bandwidth allocation method used in the ATM-PON in which a packet size is fixed. Accordingly, the GE-PON raises a bandwidth allocation scheduling problem since it is a PON having a point-to-multipoint structure, which has not been used yet in an Ethernet-based network.
Downstream traffic is broadcasted from the OLT to the ONUs, as done in the existing Ethernet. However, upstream traffics transmitted from the ONUs to the OLT are multiplexed before arriving at the OLT. In order to prevent traffic collision, the OLT must divide time so that the ONUs transmit data at a different time. Further, since the ONUs have different amounts of traffics to transmit, a dynamic bandwidth allocation is required in order to efficiently transmit the traffics to the OLT without time delay.
FIG. 2 is a block diagram illustrating an example of a GE-PON structure capable of performing the dynamic bandwidth allocation. As illustrated, the GE-PON has an OLT 20 serving as a toll center, an ODN 26 consisting of an optical splitter that is a passive element, three ONUs 22a to 22c, and their associated end users 24a to 24c. Such a GE-PON employs TDM for the upstream transmission due to its point-to-multipoint tree structure.
The GE-PON, unlike the ATM-PON of FIG. 1, forms upstream and downstream frames based on a variable-length Ethernet frame for bandwidth allocation. A variable-length Ethernet frame format and GE-PON functions related to the variable-length Ethernet frame, i.e., initial ONU registration, late ONU registration, ranging and dynamic bandwidth allocation procedures, are disclosed in Korean patent application No. 2002-2765, filed by the applicant on 17 Jan. 2002, entitled “An Operation Implementation Method in a Gigabit Ethernet Passive Optical Network System and Ethernet Frame Structure Therefor.”
The operation of the GE-PON illustrated in FIG. 2 will now be described in brief with reference to the Korean patent application No. 2002-2765. First, the ONUs 22a to 22c are registered in the OLT 20 to indicate their positions and existences. The ONUs are allocated their own unique IDs. Thereafter, a ranging, which is a distance correction procedure, is performed. Here, although the ONUs 22a to 22c can correct the synchronization error for the upstream/downstream time delay during the registration procedure, an error which may occur due to an external parameter, such as temperature, is not corrected accurately.
If the OLT 20 gives a data transmission opportunity to the ONUs 22a to 22c that have completed the ONU registration and the distance correction, through an upstream data transmission opportunity grant frame, then the ONUs 22a to 22c measure the amounts of data stored in their buffers, write the corresponding queue values in the band allocation request frames, and transmit the resultant band allocation request frames to the OLT 20. Here, the upstream data transmission opportunity grant frame is a downstream packet used when the OLT desires to give an upstream data transmission opportunity to one of the ONUs 22a to 22c, and the band allocation request frame is an upstream packet used when one of the ONUs 22a to 22c, granted by the OLT 20, sends a band allocation request to the OLT 20.
After receiving the bandwidth allocation requests for a particular time within one time slot, the OLT 20 allocates an appropriate data transmission bandwidth through scheduling. Here, allocation information of the data transmission bandwidth is comprised of a data transmission start time and a data transmission holding time. After the allocation of the data transmission bandwidth, the ONUs 22a to 22c transmit data to the OLT 20 at an allocated transmission start time during an allocated transmission holding time.
FIG. 3 is a diagram illustrating an example of a bandwidth allocation method performed by the OLT 20 of FIG. 2. Here, the bandwidth allocation between the OLT 20 and one ONU 22a among the ONUs 22a to 22c will be described.
After matching the synchronization with the ONU 22a, the OLT 20 transmits a transmission opportunity grant frame GRANT to the ONU 22a. As a result, the ONU 22a has an opportunity to transmit a request signal to the OLT 20. Therefore, when data transmission request is needed, the ONU 22a sends a bandwidth allocation request to the OLT 20 after matching the synchronization with the OLT 20. Upon receiving the bandwidth allocation request from the ONU 22a, the OLT 20 performs a scheduling for the bandwidth allocation to the ONU 22a. 
Thereafter, the OLT 20 allocates a data transmission bandwidth to the ONU 22a according to the scheduling result. After being allocated a data transmission bandwidth, the ONU 22a transmits data to the OLT 20 for a time allocated according to the allocated data transmission bandwidth.
The future standardization is expected to be developed in a direction of determining details of SLA (Service Level Agreement) and MPCP (Multi-Point Control Protocol) given that the PON is one of basic solutions for FTTH. However, no SLA procedure has been defined yet in IEEE802.3, like the existing LAN (Local Area Network) protocol.
However, according to ITU-T G.983.2, ATM-PON management specification defines several options in separating characteristics for a plurality of service class traffics without distinguishing voice data and in managing the traffics. Accordingly, from the requirement for the GE-PON, it is indicated that several multimedia services having the existing data transmission network will be performed in the Ethernet. The GE-PON requirements include a video broadcast streaming and audio broadcast streaming service as a downstream broadcasting service. In addition, the GE-PON requires a dynamic bandwidth allocation capable of accepting the characteristic of busty traffic and the acceptance of a real-time video/audio service.
However, in the existing LAN environment, particularly, in an Ethernet-based network, a CSMA/CD (Carrier Sense Multiple Access/Collision Detect) scheme is basically serviced, but the operation or measure for the quality of service is not reflected in a protocol. Given that a transfer rate of the Ethernet exceeds a Giga class and a MAC operation is no longer requires a more CSMA/CD operation, it can be understood that acceptance of various services for the GE-PON has an important meaning to a GE-PON protocol.
Meanwhile, one of the most fundamental services that the GE-PON must accept as an FTTx solution is a voice service. For the voice service, a short latency time and a low jitter noise are required during transmission.
A period for which the OLT 20 performs scheduling for bandwidth allocation to the ONU 22a is called a “time slot,” and according to GR-909, a maximum value of the time slot is defined as 2 msec in a FITL (Fiber In The Loop) system. Accordingly, when voice data having a latency time characteristic is managed in the same queue like the other VBR service, since a basic transmission time exceeds 2 msec even on the assumption that a dynamic bandwidth allocation algorithm allocates a bandwidth by giving a priority to the transmission of voice data for a voice service, the conventional dynamic bandwidth algorithm cannot accept the voice service when the size of a time slot is 2 msec.